Change Department Responsible at runtime

Hi Experts

We have enhanced a business activity and introduced a custom tab.

One of the field is follow up action. Based on the value selected in follow up action, the department responsible field needs to change.

We have tried using the below fm to enhance the code gets triggered when selecting the drop down but little success

COM_PARTNER_READ_MULTI_OB

COM_PARTNER_PUT_MULTI_OB

Please could you let me know any sutiable fm to do the same.

Thanks in advance.

NB:Please refer the attachment.